1. Field of the Invention
The present invention relates to a balun filter and a radio-frequency system, and more particularly, to a balun filter and a radio-frequency system capable of saving the entire area of the RF system.
2. Description of the Prior Art
In radio-frequency (RF) systems, signals transmitted and received by antennas are single-ended (unbalanced) signals, and signals processed by RF processing modules, following the RF systems, are differential (balanced) signals. Therefore, a balance-to-unbalance converter (Balun converter), coupled between an antenna and an RF processing module, is required to convert the balanced signals into the unbalanced signals, and vice versa. In addition, a band pass filter is usually coupled between an antenna and an RF processing module, for filtering out noise.
For example, FIG. 1 is a schematic diagram of an RF system 10 according to prior art. The RF system 10 includes an antenna 100, a band pass filter 102, a balun converter 104 and an RF processing module 106. When the RF system 10 functions as a receiver, the antenna 100 receives unbalanced RF signals from air, the band pass filter 102 filters noise outside of a specific frequency band, and the balun converter 104 converts the unbalanced signals into balanced signals. The balanced signals are inputted to the RF processing module 106 for further processing. When the RF system 10 functions as a transmitter, the RF processing module 106 generates an RF signal as a balanced differential signal. The balanced differential signal should be delivered to the balun converter 104 to be converted to an unbalanced signal. After the band pass filter 102 filter out noise, the RF signal is transmitted by the antenna 100 to the air.
In detail, as shown in FIG. 1, the balun converter 104 utilizes a rat-race coupler and a specific impedance matching resister for realizing balance-to-unbalance conversion. The band pass filter 102 includes multiple coupled lines, for filtering signals in a specific frequency band. In other words, the balun converter 104 and the band pass filter 102 are designed individually and connected together in series. However, when the balun converter 104 and the band pass filter 102 are connected in series, a problem of impedance match occurs, the system performance is reduced. In another perspective, in addition to the transmission line between the balun converter 104 and the band pass filter 102, both the balun converter 104 and the band pass filter 102 occupy a certain circuit area, such that the area where those components are disposed on is needed to be expanded. If the transmission line between the balun converter 104 and the band pass filter 102 is too long, a loss on transmission is increased, and the gain of the antenna will be reduced as well.
As can be seen from the above, in the prior art, designing the balun converter and the band pass filter individually and connecting the two together in series requires a larger area, increases the loss on transmission path, reduces antenna gain, and has impedance matching problem. Therefore, it is necessary to improve the prior art.